Many different means of security are available to prevent duplication of printed commercial documents such as special papers (water marked paper) and special inks (fluorescent inks and other optically variable inks) which form latent images or images that change color.
The use of latent images as a security measure is well known. To be useful as a security measure, latent images must be well camouflaged but readily and easily viewable to the user, preferably by a simple procedure. An example of such a latent image is described in U.S. Pat. No. 5,468,581, which is formed when printing documents using an intaglio process. The latent image is overprinted on the visible image such that the latent image is visible when the document is tilted and viewed at an angle. The latent image is caused by the variation of the slight shadow from the raised ink pattern formed by the intaglio process or other printing method which produces raised ink patterns.
Optically variable inks have been used to provide latent images and images which change color when exposed to a light source other than ambient light. These optically variable inks allow for non-destructive testing of the security feature allowing the printing of such inks to be monitored. Such optically variable inks typically contain a fluorescent compound or photochromic compound which responds to infrared or ultraviolet light. An example of an aqueous printing ink for jet printing which fluoresces under ultraviolet radiation is described in U.S. Pat. No. 4,153,593. The dyes described in this reference are water soluble and include fluorescein, eosine dyes and Rhodamine dyes.
Representative disclosures of other inks include U.S. Pat. No. 4,328,332, issued to Hayes et al. on May 4, 1982, and U.S. Pat. No. 4,150,997, issued to Hayes on Apr. 24, 1979. Kaule et al., U.S. Pat. Nos. 4,452,843 and 4,598,205, disclose rare earth metal luminophores which absorb in the visible region and optionally the near-infrared region and can be excited in substantial portions of the visible or near IR-region. Yoshinaga et al., U.S. Pat. No. 5,503,904, disclose recording media with an invisible identification mark composed of regions of high reflectance and low reflectance in the near infrared region. Near infrared coloring materials are said to comprise xanthene, oxazine, thiazine, polymethine and stryl compounds.
While the use of fluorescent inks and dyes has been effective and versatile, with the advent of today's personal computers and color copiers, conventional security measures such as these have been overcome, particularly where records are only casually inspected, such as sales receipts and transaction records. Therefore, it is desirable to provide additional security measures to supplement the fluorescent pigments and dyes.
Adding additional security measures is complicated by many factors. One is that there are many types of printing inks with compositions adapted to be employed in particular printing operations. For example, the inks for ink jet printers often must be conductive, have viscosity values within a certain range and contain no large particulate matter (below 5 .mu.m) and the ink should not dry within the ink jet over short periods of time. Jet printing processes are described in Report No. 1722-1 of the Stanford University Electronic Research Laboratory dated March 1964, entitled "High Frequency of Oscillography with Electrostatically Deflected Ink Jets", and U.S. Pat. Nos. 3,596,275, 4,269,627, 4,153,593, 4,328,332, and 4,150,997. Special ink formulations are often employed in relief printing, offset printing, intaglio printing, lithography and silk screening.
Another factor which complicates adding a security measure to a security ink is that water-based inks are preferred to minimize the impact on the environment and avoid flammable vapors during use. This limits the components that can be added to the security ink.
An additional factor which complicates adding a security measure to a security ink is that it is difficult to complement the performance of fluorescent and photochromic pigments and dyes within optically variable inks without interfering with their performance. Parameters such as these place limitations on the additives or other components which can be used with security inks, making it difficult to provide multiple security measures within a security ink.
The above factors must be considered for the inks to be printed on plain paper. Where the security features are desired for thermal paper, the ink has additional requirements due to the special thermosensitive coatings thereon which generate images when activated by heat. The inks must not pre-react the reactive components within the thermosensitive coating of the thermal paper to detract from the papers printing performance. Certain chemical factors can adversely affect and degrade the performance of the thermosensitive coating and should be avoided such as some organic solvents (ketones), plasticizers (polyethylene glycol type) amines (ammonia) and certain oils (soy oil).
Direct thermal paper is a thermosensitive recording material on which print or a design is obtained by the application of heat energy. Thermal paper comprises a base sheet and a coating, and like other coated papers, the coating is applied to give new properties to the base sheet. However, a major distinction in thermal paper from other coated papers is that special color forming chemicals and additives are present in the coatings such that when heat is applied by a thermal head, the color forming chemicals react to develop the desired print or image.
The most common type of thermal coating is the dye-developing type system. The three main color producing components in a dye developing-type thermal coating are colorless dye (color former), a bisphenol or an acidic material (color developer) and sensitizer. These solid materials are reduced to very small particles by grinding and incorporated into a coating formulation along with any optional additives such as pigments, binders and lubricants. This coating formulation is then applied to the surface of paper or other support system using various types of coloring application systems and dried. Images are formed on the coated surfaces by the application of heat to melt and interact the three color producing materials.
To protect thermal paper from environmental conditions, and premature coloration from handling, a number of developments have been made. One is to produce a barrier or protection layer on top of the thermal coating (see U.S. Pat. Nos. 4,370,370; 4,388,362; 4,424,245; 4,444,819; 4,507,669; and 4,551,738). Another approach is to encapsulate the reactive components in microcapsules which rupture or are permeable when exposed to heat. See U.S. Pat. Nos. 4,682,194; 4,722,921; 4,740,495; 4,742,043; 4,783,493; and 4,942,150. A U.V. cured silicone acrylate/methacrylate protective coating for thermosensitive layers is described in U.S. Pat. No. 4,604,635. The use of a water soluble polyvinyl alcohol based intermediate coating as a protective layer on a thermal coating is described in EP 339,670. This intermediate coating may be cured by drying, exposure to U.V., or exposure to electron beam radiation and is overcoated with an electron beam radiation- cured layer. These protective measures will not always prevent premature coloration of thermal papers when exposed to a security ink, particularly when printed on the side opposite the thermosensitive coating of the thermal paper. In addition, these protective coatings are applied uniformly to the thermosensitive coating and not in selected regions as when printing so that any minor discoloration of the thermosensitive coating by these protective coatings may be uniform.
U.S. Pat. No. 5,883,043 describes a thermosensitive recording material with a latent image on the backside thereof that can function as a security feature. The latent image comprises a pigment or dye with variable light absorption properties and a water repelling agent that renders the image waterproof. These latent images are prepared by flexographic printing with the use of a security ink which preferably contains an aqueous based solvent.